Man has engaged in war on the land, in the sea, in the air and in the electromagnetic spectrum. The electromagnetic spectrum has been used by the military for improved communications, guidance of aircraft and missiles and the navigation of ships and planes. A nation seeks control of the electromagnetic spectrum because of the military's increasing dependency on its use for surveillance of potential enemy forces, communications between military units, detection of enemy military forces and the guidance and control of aeroplanes and missiles. With a mastery of the electromagnetic spectrum, one adversary could achieve an indispensable technological advantage for conquering an enemy or discouraging a potential aggressor.
Electronic warfare is the employment of electronic devices and techniques for the purposes of determining the existence and disposition of an enemy's electronic aids to warfare and destroying or degrading the effectiveness of the enemy's electronic aids to warfare. Radar is a type of electronic warfare that utilizes beamed and reflected radio frequency (RF) energy for detecting and locating objects, measuring distances, altitude, navigating, homing, bombing, and other purposes. Radar equipment may be installed on aircraft. When radar is installed on an aircraft, a radar dome or "radome" is often used to streamline and protect the radar antenna or antennas from adverse environments. Electrically the radome must cause a minimum distortion of the electrical characteristics of the antenna radiation pattern.
High-performance radome designs may include frequency selective surfaces in their construction. The frequency selective surfaces are often constructed from conventional printed circuit RF filter elements. The frequency selective surfaces of the radomes are designed to take into account the desired bandwidth, frequency selectivity, and frequency roll-off characteristics of the radar. The mechanical parameters of the frequency selective surfaces of the radome (including overall geometry such as element size and shape) and the electrical properties (such as dielectric constants and conductivity of the various layers) are all interrelated properties which affect the performance of the frequency selective radome. In other words, the desired performance of the high performance radome is largely a function of element size and shape as well as element spacing, or distribution. For conventional frequency selective surfaces of radomes, the frequency and polarization response is dependent on the size and shape of the elements (which defines the response of the element itself) and the distribution of the elements (which defines the electromagnetic interaction between the elements). Thus, frequency selective surfaces of radomes have been designed to meet specific radome performance requirements.
Prior art radome design was mostly by cut and try methods. These methods were expensive and time consuming, since they relied on flight test data and radome failures to obtain a design.
As the prior art developed the element dimensional tolerances of frequency selective surfaces of radomes became more exacting. Computer simulations of the frequency selective surfaces of the radomes were performed to avoid costly design errors. When the simulations were performed it was assumed that all of the elements comprising a frequency selective surface were of uniform size, shape, and distribution. If the actual elements (e.g., slots) were not the same size that was simulated, the performance of the actual frequency selective surface would not be the same as modeled. Oftentimes, the actual frequency selective surface had different correlated errors in different regions of the slot array that caused distortion of the antenna pattern. These correlated errors in slot size caused phase errors which resulted in the pattern distortion and an increase in the side-lobe levels. In practice, unfortunately, the close manufacturing tolerances for the element size, shape and spacing make it exceedingly difficult to consistently construct the frequency selective surfaces of the radomes in a manner that will meet engineering design requirements as modeled.
One method utilized by the prior art for the construction of close tolerance frequency selective surface radomes was called the strip filter technique. The strip filter technique involved the producing of the frequency selective surfaces in three strips, using state-of-the-art printed circuit board technology to meet the element dimensional tolerance requirements. One of the difficulties encountered by the prior art was that it was hard to align the three strips and to join them in such a manner that the assembly would appear electrically to be one piece and not degrade aircraft low-observability characteristics. Thus, each radome would not necessarily have the same RF and RCS performance characteristics. The prior art attempted to solve the foregoing problem by filling the gaps between the three strips (during layup of the radome) with electrically conductive materials. Joining the strips by "filling the gaps" was problematic, however, for quality assurance reasons. Moreover, this joining technique did not achieve consistently satisfactory results.
Reference may be had to the following patents for further information concerning the state of the prior art.
In U.S. Pat. No. 3,761,937, issued Sept. 25, 1973 entitled "Radio Frequency Transmitting Apparatus Having Slotted Metallic Radio Frequency Windows" to Tricoles et al. there is disclosed a radome that has increased bandwidth capability. The radome may be made of a metallic sheet that has increased bandwidth capability. The radome may be made of a plurality of slots with edges that are beveled-towards each other from the outer surface of the sheet, such that the metallic walls of the window are thin and approach the skin depth for the highest frequency of the radio frequency waves which are transmitted.
In U.S. Pat. No. 3,975,738, issued Aug. 17, 1976 entitled "Periodic Antenna Surface of Tripole Slot Elements" to Pelton et al. there is disclosed an antenna system in which a conical shaped metallic radome has a surface-composed of a periodic array of radiating slot elements.
In U.S. Pat. No. 4,125,841, issued Nov. 14, 1978 entitled "Space Filter" to Munk there is disclosed a space-filter formed as a composite multi-layered structure that utilizes a periodic slot array structure nested between first and last strata of dielectric material. The filter exhibits a constant bandwidth characteristic over a broad range of angles of incident radiation.
In U.S. Pat. No. 4,126,866, issued Nov. 21, 1978 entitled "Space Filter Surface" to Pelton there is disclosed a surface that is used as a space filter in electromagnetic space. The filter surface is formed as a periodic array of recurrent filter components clustered in groups of three each incorporating pairs of elements extending outwardly at an internal angle of 120 degrees.
In U.S. Pat. No. 4,314,255, issued Feb. 2, 1982 entitled "Electromagnetic Angle Filter Including Two Staggered, Identical, Periodically Perforated Conductive Plates" to Kornbau there is disclosed an angle filter for electromagnetic radiation having a predetermined wavelength .lambda.. The angle filter includes a planar parallel pair of perforated conductive plates having arrays of periodic perforations.
In U.S. Pat. No. 4,275,859, issued Jun. 30, 1981, entitled "Optical Dome Protection Device" to Bleday there is disclosed a rain and shock protection for an optical dome.
In U.S. Pat. No. 4,352,108, issued Sep. 28, 1992 entitled "Antenna Beam Shaping Structure Employing Dipoles Arrayed On A Parabolic Surface" to Milne there is disclosed an antenna beam shaping structure which comprises a first surface which is a surface of revolution swept out by a part of a parabolic curve rotated about and being included at an angle in the range of about 70 to 80 degrees to the antenna axis with the focus of the parabolic curve substantially on the antenna axis, and a set of electrically isolated dipoles mounted on the surface and similarly oriented such that they lie along one of (i) edges of axial planes of the antenna, or (ii) edges of planes normal to the antenna axis.
In U.S. Pat. 4,388,388, issued Jun. 14, 1983 entitled "Method of Forming Metallic Patterns On Curved Surfaces" to Kornbau et al. there is disclosed a method for forming frequency selective surfaces on a curved radome shell. The slots are etched into the metallic surface on the curved radome structure.
In U.S. Pat. No. 4,467,330, issued Aug. 21, 1984 entitled "Dielectric Structures For Radomes" to Vidal et al. there is disclosed a radome which has a dielectric material which has placed within it a plurality of ring-shaped elements, each forming a completely closed loop configuration, such elements producing an inductive effect substantially equal to the capacitive effect of the dielectric material so as to match the electrical characteristics of the structure to a selected range of frequencies of electromagnetic energy which is to be transmitted therethrough.
In U.S. Pat. 4,570,166 issued Feb. 11, 1986 entitled "RF-Transparent Shield Structures" to Kuhn et al. there is disclosed a dielectric plug that is used as an aid to protection against severe environmental conditions, especially as encountered in missile applications.
In U.S. Pat. No. 4,574,288, issued Mar. 4, 1986 entitled "Passive Electromagnetic Wave Duplexer For Millimetric Antenna" to Sillard et al. there is disclosed a passive duplexer for electromagnetic waves operated within the millimetric wave range.
In U.S. Pat. No. 4,684,954, issued Aug 4, 1987 entitled "Electromagnetic Energy Shield" to Sureau et al. there is disclosed a radome shutter structure which is placed in a closed or shut position when the diodes are biased in a forward or conductive state and is placed in an open condition when the diodes,are reversed biased or in a non-conductive state.
In U.S. Pat. No. 4,785,310, issued Nov. 15, 1988 entitled "Frequency Selective Screen Having Sharp Transition" to Rosen there is disclosed a frequency selective screen (18) that is employed as a diplexer to separate each of one or more-radio frequency signals into first and second bands of frequencies by allowing the first band of frequencies to pass therethrough and reflecting the second band of frequencies.
In U.S. Pat. No. 4,786,915, issued Nov. 22, 1988 entitled "Attenuation of Microwave Signals" to Cartwright et al. there is disclosed a membrane of absorbent plastic that is stretched across the aperture of a dish reflector.
In. U.S. Pat. No. 4,864,321, issued Sep. 5, 1989 entitled "Electromagnetic Energy Shield" to Sureau there is disclosed a structure for transmitting electromagnetic energy within a selected frequency range and preventing such transmission outside such range in which an insulative member has a metallized surface which includes an array of non-metallized regions each having the shape of a Jerusalem cross, the vertical and horizontal cross arms thereof having metallized regions along their length to form non-metallized gaps with the edges thereof.
In U.S. Pat. No. 4,970,634, issued Nov. 13, 1990 entitled "Radar Transparent Materials" to Howell et al. there is disclosed a structure for reflecting visible light, herein the structure is formed of a low dielectric constant material having an external surface comprising an electrically conductive layer of material which has an array of slots therein so as to be as substantially transparent to microwave radiation of a predetermined wavelength which will impinge upon the structure during use, while being reflective to light.
In U.S. Pat. No. 5,103,241 issued Apr. 7, 1992 entitled "High Q Bandpass Structure For The Selective Transmission and Reflection of High Frequency Radio Signals" to Wu there is disclosed a multi-layered structure that incorporates two separate frequency selective surfaces in a parallel spaced relationship. The frequency selective surfaces are embedded in rigid dielectric layers.
In U.S. Pat. No. 5,140,338 issued Apr. 7, 1992 entitled "Frequency Selective Radome" to Schmier et al. there is disclosed a two pole frequency selective surface for passing electromagnetic wave energy in a selected frequency band.